In a modern optical communications network, multiple optical carriers transport digital traffic between access multiplexers at the edges of the network and photonic switch nodes located at strategic points within the core of the network. The link between a particular access multiplexer and a particular photonic switch node may be adapted to run only one wavelength per fiber or it may adhere to a multi-wavelength carrier frequency plan with typically 400–500 GHz spacing (referred to as “Sparse DWDM” since, although the individual optical carriers are generated with the required stability for DWDM transmission, they are widely spaced to create a known sparse population of the DWDM grid). Despite the low concentration of optical carriers on a given link, however, each modulated optical carrier transmitted by the access multiplexer has to appear at a precisely controlled optical frequency. This is because upon receipt at the photonic switch node, signals may be multiplexed together by a process of interleaving into a true DWDM stream for transmission through the core DWDM trunking network to other access multiplexers or to a core node router without undergoing any wavelength conversion.
The set of acceptable wavelengths for the optical carriers is known as the interoffice trunking wavelength plan, which has a narrower grid in order to achieve the large payload capacity of a high number of optical carriers on each fiber. This spacing is generally on the order of 100–200 GHz or less, and 100 GHz will be assumed here for simplicity. To facilitate interoperability, interoffice trunking wavelength plans are typically specified by the International Telecommunications Union (ITU). In order for a modulated optical carrier to be transmittable from one access multiplexer directly across a DWDM network to another access multiplexer or core node router without undergoing wavelength conversion, the optical carrier has to be precise to a small part of the DWDM grid, possibly to within at +/−1–3 GHz for a 100 GHz grid, and even tighter tolerances for a closer optical grid spacing.
A conventional approach to providing precisely controlled optical signal sources would consist of placing very precise and necessarily tunable optical sources at each access multiplexer. However, this is not only expensive, but is especially difficult to implement due to the location of the access multiplexers and their isolation from any reference, requiring it to make use of a self-contained and necessarily tunable or provisionable high precision source. Thus, the solution is in this case expensive and unreliable, as the number of sources scattered throughout the network is very large and thus the probability of a malfunction or mis-programming of a remoted function is higher.
If, on the other hand, unmodulated optical carriers were distributed to the access multiplexer from a centralized source, only to be turned around and modulated before being sent to the photonic switching node, then it is conceivable that all the necessary optical carriers could be generated at a single location under tightly controlled conditions and assembled into the necessary groups for distribution in specific appropriate groups to match access architectures, modularities, optical carrier plans, etc. This would permit the generation of optical wavelengths that are sufficiently precise in optical frequency such that optical carriers received at the photonic switch nodes could be directly coupled into the interoffice trunking wavelength plan, as required.
Thus, in a photonic switch node hosting, for example, 500 access-side optical carrier ports, each potentially associated with an access multiplexer, and utilizing a 5 phase, 8 channel sparse-DWDM plan mapping over a 40-channel interoffice trunking wavelength plan, 40 centrally located optical sources with appropriate buffering, amplification and splitting could do the work of 500 tunable sources further out in the access multiplexers. Furthermore, the technical requirements for locking 40 devices would be far less complex and far less costly than those for an individual tunable optical carrier locking system at each access multiplexer. Clearly, therefore, economies of scale can be achieved by distributing the wavelengths from a central point. In addition, the wavelengths could be generated in a benign environment and could be readily locked to grid, including locked to any reference wavelength distributed as a network master reference.